The present invention relates to a thermally-assisted magnetically recording method and a thermally-assisted magnetic recorder, and more particularly, to a novel method for thermally-assisted magnetic recording and a thermally-assisted magnetic recorder, capable of heating a magnetic recording medium by a heat source to magnetically record data to the medium with an extremely high density.
Magnetic recorders for magnetically recording and reproducing information are under continuous development as large-capacity, high-speed and inexpensive information storage means. Especially, recent hard disc drive (HDD) has shown remarkable improvements. As proved on the product level, its recording density is over 10 Gbpsi (gigabits per square inch), internal data transfer rate is over 100 Mbps (megabits per second) and price is as low as several yens/MB (megabytes). The high recording density of HDD is due to a combination of improvements of a plurality of elements such as signal processing technique, servo control mechanisms, head, medium, HID, etc. Recently, however, it has become apparent that the thermal agitation of the medium disturbs the higher density of HDD.
The high density of magnetic recording can be attained by miniaturizing the recording cell (recording bit) size. However, as miniaturization of the recording cell progresses, the signal magnetic field intensity from the medium is reduced. So, to assure a predetermined signal-to-noise ratio (S/N ratio), it is indispensable to reduce the medium noise. The medium noise is caused mainly by a disordered magnetic transition. The magnitude of the disorder is proportional to a flux reversal unit of the medium. The magnetic medium uses a thin film formed from polycrystalline magnetic particles (referred to as xe2x80x9cmultiparticle thin filmxe2x80x9d or xe2x80x9cmultiparticle mediumxe2x80x9d herein). In case a magnetic exchange interaction works between magnetic particles, the flux reversal unit of the multiparticle thin film is composed of a plurality of exchange-coupled magnetic particles.
Heretofore, when a medium is to have the recording density of several hundreds Mbpsi to several Gbpsi, for example, noise reduction of the medium has been attained mainly by reducing the exchange interaction between the magnetic particles and making smaller the flux reversal unit. In the latest magnetic medium of 10 Gbpsi in recording density, the flux reversal unit is of only 2 or 3 magnetic particles. Thus, predictably, the flux reversal unit will be reduced to the size of only one magnetic particle in near future.
Therefore, to ensure a predetermined S/N ratio by further reducing the flux reversal unit, it is necessary to diminish the size of the magnetic particles. Taking the volume of a magnetic particle as V, a magnetic energy the particle has can be expressed as KuV where Ku is a magnetically anisotropic energy density the particle has. When V is made smaller for a lower medium noise, KuV becomes smaller with a result that the thermal energy near the room temperature will disturb information written in the medium, and reveals the problem of thermal agitation.
According to the analysis made by Sharrock et al., if the ratio between magnetic energy and thermal energy (kT, where k is Boltzman""s constant and T is absolute temperature) of a particle, KuV/kT, is not 100 or so, it will impair the reliability of the record life. If reduction of the particle size is progressed for a lower medium noise with the anisotropy energy density Ku being maintained at (2 to 3)xc3x97106 erg/cc of the CoCr group alloy conventionally used as a magnetic film in the recording medium, it is getting difficult to ensure a thermal agitation resistance.
Recently, magnetic film materials having a Ku value more than 107 erg/cc, such as CoPt, FePd, etc., have been attracting much attention. However, simply increasing the Ku value for compatibility between the small particle size and thermal agitation resistance will lead to another problem. The problem concerns the recording sensitivity. Specifically, as the Ku value of the magnetic film of a medium is increased, the recording coercive force Hc0 of the medium (Hc0=Ku/Isb; Isb is the net magnetization of the magnetic film of the medium) increases, and the necessary magnetic field for saturation recording increases proportionally to Hc0.
A recording magnetic field developed by a recording head and applied to the medium depends upon a current supplied to a recording coil as well as upon a recording magnetic pole material, magnetic pole shape, spacing, medium type, film thickness, etc. Since the tip of the recording magnetic pole is reduced in size as the recording density is higher, the magnetic field developed by the recording head is limited in intensity.
Even with a combination of a single-pole head that will develop a largest magnetic field and a vertical medium with a soft-magnetic backing, for example, its maximum recording field is only around 10 kOe (Oe: oersted). On the other hand, to ensure a sufficient thermal agitation resistance with a necessary particle size of about 5 nm for a future high-density, low-noise medium, it is necessary to use a magnetic film material having a Ku value of 107 erg/cc or more. In this case, however, since the magnetic field intensity necessary for recording to the medium at a temperature approximate to the room temperature is over 10 kOe, recording to the medium is disabled. Therefore, if the Ku value of the medium is simply increased, there will arise the problem of the recording to the medium being impossible.
As having been described in the foregoing, in the magnetic recording using the conventional multiparticle medium, noise reduction, thermal agitation resistance and higher recording density are in a trade-off relation with each other, which is an essential factor imposing a limit to the recording density.
A thermally-assisted magnetic recording system will be able to overcome this problem. Preferably, such a thermally-assisted magnetic recording system using a multiparticle medium uses magnetic particles as fine as sufficiently reducing noise and uses a recording layer exhibiting a high Ku value near the room temperature in order to ensure a thermal agitation resistance. In a medium having such a large Ku value, since the magnetic field intensity necessary for recording exceeds the intensity of a magnetic field developed by the recording head near the room temperature, recording is not possible. In contrast, in the thermally-assisted magnetic recording system, locating a medium heating means such as light beams near the recording magnetic pole and locally heating the recording medium during recording to lower Hc0 of the heated portion of the medium below the magnetic field intensity from the recording head, and recording is effected.
Important points for realizing this basic concept are: recording should be completed by supplying a recording magnetic field during heating or before the heated medium cools down; only a limited area as small as the width of the recording pole should be selectively heated to prevent that adjacent tracks are undesirably heated and adjacent magnetic transition is destructed by thermal agitation.
In a mode using a multiparticle medium, in addition to thermal agitation of adjacent tracks, it is necessary to ensure that magnetic transition created in a track to be recorded does not give influences of thermal agitation to a downstream region which does not yet cool down sufficiently. However, it has the advantage that the recording density is determined by the particle size, and flux reversal speed is remarkably high.
On the other hand, a system using a continuous magnetic film, i.e. amorphous magnetic film, has shortcomings, not involved in multiparticle systems, that the recording density is determined by the thickness of the magnetic domain wall (10xe2x88x9220 nm) and, when accompanied with displacement of the domain wall, the speed of the domain wall displacement (tens of m/s) determines the data transfer speed. However, volume V of the magnetic particles can be regarded infinite, the problem of thermal agitation is out of problem. Also the system using a continuous magnetic film is equivalent to the multiparticle system in the respect of adjusting the coercive force of the medium near the room temperature higher than the head magnetic field and adjusting the coercive force of the medium of the heated portion lower than the head magnetic field.
A prior art technique trying thermally-assisted magnetic recording by using a magneto-optic medium as a continuous magnetic film is disclosed in, for example, Journal of the Magnetics Society of Japan vol. 23, No. 8, pp-1901-1906. Since this prior art uses far-field light beams as the heat source for heating a medium and locating a recording pole and the beam source in confrontation with the medium, double surface recording is impossible and near-field light cannot be used. Further, recording bit length is determined by the magnetic head, but since the recording track width is determined by the light spot, the limit of the track width is restricted by the spot size of the far-field light. That is, even when combining a short-wavelength laser and a high NA lens, the limit of the track width is hundreds of nm.
Moreover, since the light-irradiated position and the recording field applying position substantially coincide in that prior art, it has also the problem that data transfer speed is determined by the time required for heating the medium.
As explained above, the use of a multiparticle medium in a thermally-assisted magnetic recording system involves various problems, namely, uncertainty of means for moving a medium across a recording magnetic field just when the medium is sufficiently heated, difficulty of realization of a compact, light and inexpensive thermally-assisted magnetic recording head, and particularly in a mode using near-field light, lack of appropriate positional relation between the size of the optical aperture or collector and the recording magnetic pole.
On the other hand, the use of a continuous magnetic film medium involves the problems: double-surface recording being impossible, insufficient recording density due to impossibility of using near-field light, data transfer speed being restricted by the thermal response of the medium because of coincidence between the light-irradiated position and the position where the recording magnetic field is applied.
The invention has been made under the knowledge about those problems. That is, it is an object of the invention to provide a thermally-assisted magnetic recording method and a thermally-assisted magnetic recorder capable of optimizing the timing of heating the medium and magnetic recording during thermally-assisted magnetic magnetic recording for recording magnetic information by heating the medium with a heat source such as light beams, and thereby drastically improving the recording density while enabling miniaturization, reduction in weight and higher reliability.
According to the invention, there is provided a thermally-assisted magnetic recording method for first heating a recording medium to decrease the coercive force of a recording portion and thereafter applying a recording magnetic field from a recording magnetic pole onto the recording portion decreased in coercive force to enable magnetic recording of information, characterized in: a reversing point of magnetization where the coercive force of the recording portion equals the intensity of the recording magnetic field being located in a position in the leading side of the trailing edge of the recording magnetic pole.
In the thermally-assisted magnetic recording method, the relation of Dxe2x89xa6Bmin is preferably satisfied, where D is the distance from the reversing point of magnetization and the trailing edge of the recording magnetic pole, and Bmin is the minimum magnetic transition distance recorded on the recording portion.
According to the invention, there is further provided a thermally-assisted magnetic recorder comprising: a heat source for heating a recording portion of a recording medium; and a recording magnetic pole for recording magnetic information by applying a recording magnetic field to the recording portion heated by the heat source and decreased in coercive force,
a reversing point of magnetization where the coercive force of the recording portion equals the intensity of the recording magnetic field being located in a position in the leading side of the trailing edge of the recording magnetic pole.
In the thermally-assisted magnetic recorder, the relation of Dxe2x89xa6Bmin is preferably satisfied, where D is the distance from the reversing point of magnetization and the trailing edge of the recording magnetic pole, and Bmin is the minimum magnetic transition distance recorded on the recording portion.
The thermally-assisted magnetic recorder may further comprises a magnetic reproducing element located in a position in the trailing side of the recording magnetic pole to detect the magnetic information recorded on the recording portion.
In the thermally-assisted magnetic recorder, the relation of Dmrxe2x89xa6vxc2x7xcex94Txe2x89xa6Dmr+Lmag is preferably satisfied, where Lmag is the distance from the leading edge to the trailing edge of the recording magnetic pole, Dmr is the distance from the trailing edge of the recording magnetic pole to the center of a magnetic detector of the magnetic reproducing element, xcex94T is the time interval from the moment of reverting the recording magnetic field for recording magnetic transition on the recording portion to the moment for the magnetic reproducing element to detect the magnetic transition recorded on the recording portion, and v is the relative velocity between the recording portion and the recording magnetic pole.
In the thermally-assisted magnetic recorder, the heat source may be an electron emitter which emits electrons to the recording medium to heat the recording portion.
In the thermally-assisted magnetic recorder, the heat source may be a light emitting element located in a position in the leading side of the recording magnetic pole, and the relation of Dthxe2x89xa64L is preferably satisfied, where Dth is the distance from the trailing edge of an emitting portion of the light emitting element to the leading edge of the recording magnetic pole, and L is the distance from the leading edge to the trailing edge of the emitting portion.
According to the invention, there is further provided a thermally-assisted magnetic recorder comprising: a light emitting element as a heat source for heating a recording portion of a recording medium; and a recording magnetic pole located in a position in the trailing side of the light emitting element to record magnetic information on the recording portion of the recording medium heated by the light emitting element by applying a recording magnetic field,
wherein before the recording portion heated by the heat source passes through sad recording magnetic field applied by the recording magnetic pole, there is provided a moment where the coercive force of the recording portion is smaller than the recording magnetic field, and the relation of Dthxe2x89xa64L is satisfied, where Dth is the distance from the trailing edge of an emitting portion of the light emitting element to the leading edge of the recording magnetic pole, and L is the distance from the leading edge to the trailing edge of the emitting portion.
In the thermally-assisted magnetic recorder, the recording magnetic pole is preferably buried monolithically in a position in the trailing side of the light emitting element.
In the thermally-assisted magnetic recorder, the light emitting element may be a laser element having a fine hole, and the emitting portion is the fine hole.
The Inventors of the present invention propose a thermally-assisted magnetic recorder based on a novel concept to attain the above object. In this thermally-assisted magnetic recorder, magnetic particles so fine that noise therefrom is sufficiently small are used and a recording layer having a high Ku value at a temperature near the room temperature is used to ensure a thermal agitation resistance. In a medium having such a large Ku value, since the magnetic field intensity necessary for recording exceeds the intensity of a magnetic field developed by the recording head under a temperature near the room temperature, recording is not possible. However, by locally heating the recording medium by an appropriate means, the Hc0 value of the heated portion of the medium can be reduced to below the magnetic field of the recording head to enable recording.
The recording medium may be heated by using light beams or electron beams.
For emitting electrons, any electron emitter of various types such as field emission type, thermoelectronic emission type, etc. may be used. The xe2x80x9cfield emission typexe2x80x9d is such that by providing a high potential gradient (electric field) on an electron emission surface, electrons are directly emitted from the surface. Especially when the present invention adopts a field emission type electron emitter, since the electron emission area is on the order of 10 nm, an area of about 10 nm of the medium can easily be heated, thus the present invention can attain a resolution far beyond that of the conventional method using light beams. However, in case of an electron emitter of the thermoelectronic emission type being used, substantially the same effect is still obtained by converging the electron beam to a predetermined size.
As having been described in the foregoing, according to the present invention, a low-noise multiparticle medium formed from very fine particles, necessary for high density magnetic recording and reproduction, can be made to have a sufficiently high resistance against the thermal agitation at a temperature near the room temperature, and the coercive force of the medium, that is, a necessary magnetic field for a flux reversal, is reduced by irradiation of light beams or electron beams onto a portion of the medium to which a recording magnetic field is applied, to thereby enable a practical thermal characteristic head to attain a high speed of recording.
Further, the invention enables the use of near-field light by supplying both light beams as the heat source and recording magnetic field from a common side of the medium, and thereby enables selective heating of a region as fine as tens of nm that cannot be realized with far-field light.
According to the invention, since the light emitting element and the magnetic recording element form an integral structure, excluding the optical system having a complicated structure and a heavy mass, the invention ensures high-speed seek operation by the head, much higher efficiency of the use of light as compared with light irradiation by using a waveguide or a fiber, and the use of a semiconductor laser of tens of mW.
According to the invention, by sequentially stacking the light emitting element and the recording magnetic pole from the downstream (leading) side of moving direction of the medium to closely locate the light emitter and the recording magnetic pole, a recording magnetic field can be applied when Hc0 of the medium has become sufficiently low.
Furthermore, by limiting the positional relation between the heat source and the recording magnetic pole within a unique range, useless flux reversal of the recording section by the recording magnetic field can be prevented.
That is, according to the invention, there can be provided a thermally-assisted magnetic recorder realizing a new concept that information can be recorded with a drastically higher density than with the conventional recorder. Thus the present invention is very advantageous in the field of art.